Contributions of pulmonary perfusion and ventilation to heterogeneity in V̇A/Q̇ measured by PET

Abstract

To estimate the contributions of the heterogeneity in regional perfusion (Q̇) and alveolar ventilation (V̇A) to that of ventilation-perfusion ratio (V̇A/Q̇), we have refined positron emission tomography (PET) techniques to image local distributions of Q̇ and V̇A per unit of gas volume content (sQ̇ and sV̇A, respectively) and V̇A/Q̇ in dogs. sV̇A was assessed in two ways: 1) the washout of 13NN tracer after equilibration by rebreathing (sV̇A(i)), and 2) the ratio of an apneic image after a bolus intravenous infusion of 13NN-saline solution to an image collected during a steady- state intravenous infusion of the same solution (SV̇A(p)). sV̇A(p) was systematically higher than sV̇A(i) in all animals, and there was a high spatial correlation between sQ̇ and sV̇A(p) in both body positions (mean correlation was 0.69 prone and 0.81 supine) suggesting that ventilation to well-perfused units was higher than to those poorly perfused. In the.prone position, the spatial distributions of sQ̇, sV̇A(p), and V̇A/Q̇ were fairly uniform with no significant gravitational gradients; however, in the supine position, these variables were significantly more heterogeneous, mostly because of significant gravitational gradients (15, 5.5, and 10%/cm, respectively) accounting for 73, 33, and 66% of the corresponding coefficient of variation (CV)2 values. We conclude that, in the prone position, gravitational forces in blood and lung tissues are largely balanced out by dorsoventral differences in lung structure. In the supine position, effects of gravity and structure become additive, resulting in substantial gravitational gradients in sQ̇ and sV̇A(p), with the higher heterogeneity in V̇A/Q̇ caused by a gravitational gradient in sQ̇, only partially compensated by that in sV̇A.